1. Field of the Invention
This invention relates to a semiconductor device and, for example, this invention relates to the structure of MOS transistors formed on an SOI (Silicon On Insulator) substrate.
2. Description of the Related Art
The SOI structure is conventionally well known as the structure having a silicon layer formed on an insulating film. The parasitic capacitance of a semiconductor element formed on the SOI structure is small and the operation speed thereof can be made high. Therefore, it is expected that the performance of LSIs is further enhanced by using the SOI structure.
However, a MOS transistor formed on the SOI structure causes a peculiar operation phenomenon in some cases because the potential of a body region in which the channel is formed is set in an electrically floating state. The phenomenon is called a floating body effect. The floating body effect has a problem that variations in the threshold voltage and leak current in the semiconductor element are caused. Therefore, various proposals are made to suppress the floating body effect. The proposals are disclosed in Jpn. Pat. Appln. KOKAI Publication No. H6-275630, Jpn. Pat. Appln. KOKAI Publication No. H7-66411, Jpn. Pat. Appln. KOKAI Publication No. H8-213564 and Jpn. Pat. Appln. KOKAI Publication No. H9-252130, for example.
Further, a comb structure is widely used for an analog CMOS circuit. By using the comb structure, MOS transistors excellent in the current supplying ability can be formed to save the space. However, when the comb structure is applied to the SOI structure, the way how to suppress the floating body effect has not been conventionally proposed.